Conventionally, a fuel pump control device as a load control apparatus is provided in a vehicle and configured to output a diagnostic signal to the exterior of the fuel pump control device. For example, such a fuel pump control device is configured to output the diagnostic signal, which indicates high level in a state of normally operating and low level in a state where malfunction occur. In such a conventional structure provided with such a fuel pump control device, an engine electronic control unit (engine ECU) may be provided to receive such a diagnostic signal. In this case, the engine ECU may be capable of detecting occurrence of malfunction in the fuel pump control device based on the diagnostic signal. However, the engine ECU may not be capable of recognizing an operating state of the fuel pump control device based on the diagnostic signal.
JP-A-11-263240 discloses an example of performing malfunction diagnosis for an apparatus, which is provided for controlling a motor in a vehicle. In such a structure, the fuel pump control apparatus may be mounted on a flange of a fuel pump module inside a vehicle. In this case, it is difficult to make contact a probe directly with the control apparatus from the exterior of the vehicle in order to perform a malfunction diagnosis, unless a service hole or the like is provided in the vehicle body. Therefore, in general, a diagnostic signal is outputted to and monitored by an engine ECU or the like. Conventionally, it is hard to detect an operating state of the motor for a pump, without monitoring a diagnostic signal. Here, when the number of signal wires for outputting signals increases according to increase in the number of the monitored objects, another problem may occur. In JP-A-11-263240, an H-bridge inverter circuit is provided for manipulating a motor for a power steering. In this structure, voltage applied to the motor is determined based on a difference, which is obtained by A/D-converting each terminal voltage. The obtained voltage is compared with a drive instruction signal value, thereby various operating states are detected. However, in JP-A-11-263240, A/D conversion of each terminal voltage and a microcomputer for calculating the difference obtained by the A/D conversion are required. Accordingly, an operating state cannot be easily distinguished. The above problems may occur not only in a motor for a fuel pump, but also in other various loads in a structure having a constraint in arrangement.